focus on low vision

Prism Primer

Alex Yoho, ABOM

Prism is generally used in optical to achieve binocular vision when the eyes aren’t working together. In the low vision world, however, eyes may work perfectly well together, but patients with various vision deficiencies may require prisms for other reasons.

Magnification is the most common method of dealing with low vision. If the image perceived by the visual system is not clear, making it larger can allow the patient to view a smaller image, even if it’s still blurred.

HIGH-PLUS POWERS

One way of achieving this is with high-plus spectacle lenses, which is usually the patient’s initial idea of the answer to their problem. What they don’t know, however, is that these lenses also dramatically shorten the working distance.

Another potential problem with high-plus lenses is that holding something that close makes maintaining binocular vision nearly impossible. Try holding your finger an inch from your nose for a few minutes, and you’ll see how difficult it is to keep your eyes in position.

As a result, there are different methods for using high-plus lenses for near magnification. The simple way is to just use one eye at a time so that the eyes don’t have to converge, thus reducing fatigue. Often, the best eye is used and the other is occluded. With super strong lenses, this may be the only option.

If both eyes have about the same visual ability, using them both can make a significant difference. In this case, in order to reduce the fatiguing convergence demand, prism base is incorporated in each eye, usually in the form of prismatic half eyes. These are usually produced in spherical powers, from +4.00D to +12.00D. The +4.00D pair would be made with 6.00Δ in each eye, +6.00D with 8.00Δ, up to +12.00D with 14.00Δ. It is imperative to keep the size of the frame to a minimum, as +12.00D lens with 14.00Δ, for example, can be more than a half-inch thick in the nasal side in an average half-eye frame.

There are custom options as well. For example, incorporating cylinder to correct astigmatism will often help. Using high-index and AR can dramatically improve both appearance and weight. Some patients may even respond to certain contrast-enhancing filter tints.

Below, an example of the Peli expansion prism at seen at varying angles. Fresnel Prism inserts can also be used

ADDRESSING SCOTOMA

Prisms can also help with scotoma. These patients’ non-seeing areas can be due to problems in the retina or the brain. For retinal deficiencies, a low vision specialist may prescribe yoked prism to shift the image to a better position on the retina while allowing the patient to relax the direction of gaze more. Prism used in this manner can be relatively low.

With brain damage such as an occipital stroke, hemianopsia can occur. This causes a vision loss in half the visual field in both eyes. For example, the patient may lose vision on the right side of both eyes, causing him to bump into things on the right. This would be an example of homonymous hemianopsia that affects half of the visual field, right, left, top, or bottom.

Heteronymous hemianopsia would affect opposite sides in each eye. Quadrantanopia affects only one quadrant in each eye.

With a prism’s ability to displace an image, there have been several methods developed to allow the patient to achieve awareness in the area of concern. The simplest is Fresnel press-on prisms. These are valuable to determine if there is potential for using other methods, or can be used by themselves. The prism is placed with base direction to the scotoma and the apex near the usable visual field. This will have to be experimented with, hence the advantage of a removable prism.

OTHER OPTIONS

Once the Fresnel prism is working—requiring experimentation and fine tuning as the patient’s scanning abilities develop—replacing it with a small optical-quality prism may be advantageous. These can be segments cemented to the lens surface or a lens inserted through the main carrier lens. Prisms around 20Δ or less can also incorporate the patient’s prescription, including astigmatism.

Success varies, depending on patient aptitudes in using the prism. Because they must develop totally new habits in using their eyes, some patients will not adapt at all. But those who do often benefit greatly from these clearer prisms.

Yet another method, developed by Eli Peli, MSc, OD, requires the patient to gaze toward the prism until their usable visual field is looking through it. A narrow strip of prism is placed above and below the eye (and sometimes one or the other), leaving about 12mm of normal lens in front of the eye. These prisms are usually much stronger and reach farther into the non-seeing area. The close proximity to the eye allows the patient constant awareness without sacrificing the ability to scan in a normal way.

Hopefully, this introduction to using prism will help your partially sighted patients get the help they need. We hope it will also encourage you to explore the world of low vision yourself. EB